Action handler is not called on a route with middleware unless a loader is also present

[carlosalbertolajr]

Reproduction

I have created a minimal reproducible example in a GitHub repository that demonstrates the issue.

Link to Repository:
https://github.com/carlosalbertolajr/react-router-middleware-with-action-bug

Steps to Reproduce:

1. Clone the repository, then run npm install and npm run dev.
2. Navigate to the /action-and-middleware route in your browser.
3. Open the terminal where the development server is running to view the logs.
4. Click the "Button 1" on the page.
5. Observe the server logs.

System Info

System:
    OS: macOS 15.6.1
    CPU: (8) arm64 Apple M2
    Memory: 108.73 MB / 8.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.20.0 - /usr/local/bin/node
    npm: 11.6.1 - /Users/caju/.npm-global/bin/npm
  Browsers:
    Chrome: 141.0.7390.56
  npmPackages:
    react-router: ^7.9.2 => 7.9.3

Used Package Manager

npm

Expected Behavior

When the form on the /action-and-middleware page is submitted, I expect to see log messages from both the middleware and the action in my server terminal.

The console output should look like this:

POST /action-and-middleware hello from middleware
POST /action-and-middleware hello from action

Actual Behavior

When the form on the /action-and-middleware page is submitted, only the log message from the middleware is printed to the console. The action function is never executed.

The actual console output is:

POST /action-and-middleware hello from middleware

This behavior seems to be a bug, as the action handler should run independently of whether a loader is present. The issue is resolved if an empty loader function is added to the route, as demonstrated by the /loader-and-action-and-middleware route in the reproduction repository.

[Aulos]

I have the same problem.

One of my actions just stopped being executed and the only thing I did was update to 7.9.3. It was working fine in 7.9.2. It was a resource route with just a middleware and an action. After adding a dummy loader it started working again 🤔

[Aulos]

I believe the issue was introduced in #14393 - isMiddlewareOnlyRoute does not take into account action being present.

[jokelbaf]

I'm experiencing the issue as well. Route with a component and action silently fails, while action-only route throws a TypeError: Value is not JSON serializable here (Because processed is undefined).